I would suggest you to run a malware and virus scan on your system. MalwareBytes is a very good anti-malware and Avast Free is a very good anti-virus. They are both freely available at www.malwarebytes.com and www.avast.com

Thanks for the help. I don't seem to have the problem now, but it has come and gone before. I had already done the scans. It doesn't seem to interfere with using the browser, but it makes me very uncomfortable. It's strange that it hasn't shown up in the scans. I think it is hiding in the browser software. It is also odd that Chrome and FireFox are affected and not IE.

Hello stemah,

I am glad to hear that your issue has been resolved. As to why the event occurs only with Firefox and Google Chrome, I guess that it is due to the browsers using similar formats or Add-Ons or maybe Internet Explorer has an additional security features that the other browsers don't have...

I wish you a very nice day and I would greatly appreciate if you could mark this problem has fixed and perhaps mark my reply as a solution or 'helpful' so that I can get some points for helping you out.

Thank you for your patience and all your interest in Mozilla Firefox!